Smokeless powder



United States Patent 2,988,436 SMOKELESS POWDER John J. ONeill, Jr., Roxana, Ill., assignor to Olin Mathieson Chemical Corporation, a corporation of Virginia No Drawing. Filed Jan. 21,1953, Ser. No. 332,517

5 Claims. (Cl. 52-11) This invention relates to explosives and more particularly to a method for making a propellant and to a novel modified propellent grain.

Conventional methods for extruding powder grains have ordinarily involved colloiding the nitrocellulose base with a volatile solvent mixture to form an extrudable dough. Such processes and particularly processes for extruding giant grains of powder from previously formed gelatinized sheets, have the disadvantage that huge presses, rolls and other equipment capable of producing extremely high pressures are required and the operation is, at best, very slow and hazardous, requiring many precautions to avoid explosion during the colloiding process. Furthermore, elaborate and expensive techniques are required for removing the volatile solvent from the extruded powder grains and, when the composition contains nitroglycerine, a large percentage of the nitroglycerine may be lost by volatilization during the extensive rolling and kneading operations. In addition, the process is batchwise and a large quantity of scrap is produced particularly in extruding large grains from previously formed sheets because the last material from each sheet as well as the first material from the next sheet to pass through the die are not useable.

Because of the deficiencies of heretofore known extrusion processes, large powder grains have ordinarily been formed by casting processes. One process wherein pellets of double base powder having their surface rendered tacky by means of a plasticizer are compressed and heated to form a powder grain is disclosed in U.S. Patent 2,417,090, issued to Silk and Dintlemann. Although such a process is well suited for forming certain types of large grains, it is not well adapted for the manufacture of grains of irregular configuration or single base grains and it requires the use of pressure with the attending hazards. Moreover, a ballistic modifier cannot be uniformly dispersed throughout the powder grain during the casting process of Silk et al. because the pellets of powder from which the large grain is formed are not dissolved but are only softened and coagmented together. This deficiency in the casting process has been partially compensated for by incorporating the modifier in the pellets of double base powder but this expedient is unsatisfactory for incorporating water soluble modifiers such as ammonium perchlorate in the double base powder grains formed from lacquer globules suspended in water.

It is therefore an object of this invention to provide an improved process for forming powder grains. It is another object of this invention to provide a method for forming powder grains having a ballistic modifier substantially uniformly dispersed throughout their composition. ,Another object of this invention is to provide a method for forming propellent powder grains of either single or double base composition having a ballistic modifier dispersed uniformly therein by a process which does not involve the use of volatile solvents or rolling, kneading and other high pressure devices. Still another object is to provide a process for making homogeneous powder grains of any size and particularly large grains having a diameter of one-half inch to several feet or more which contain a modifier. A further object of this invention is to provide a method for forming powder grains from pellets of gelatinized nitrocellulose in which a modifier which is insoluble as well as one which is soluble in Patented June 13, 196

the solvent utilized for preparing the moldable mass can be uniformly dispersed throughout the grain during the forming process. A still further object is to provide a novel homogeneous powder grain having ammonium perchlorate dispersed therein.

The foregoing objects as well as others which will become apparent from the following description are accomplished, generally speaking, in accordance with this invention by forming powder grains from a powder base utilizing a solvent whose dissolving action is for practical purposes not initiated until either heated or in contact with the powder base for appreciable periods of time.

The mixture of solvent and powder base is described hereamples of such ballistic modifiers include lead a zide, ammonium perchlorate, dye, lead, lead stearate, lead oxide, powdered iron, lead sulfide, lead chromate, alumi num dust, amonium nitrate and the like. Preferably, the plastisol dispersion is a mobile paste prepared by mixing particles of nitrocellulose with a relatively non-volatile liquid plasticizer therefor. When the nitrocellulose dissolves in the plasticizer, the resulting product is a solid gelatinized plastic mass at ordinary room temperatures.

The powder grains are shaped in accordance with this invention by any suitable means such as extruding the plastisol dispersion through a heated die and cutting the resulting strings into suitable lengths; or, the powder grains may be shaped by pouring the plastisol dispersion into a mold or other container and maintaining this mass in the desired shape until the mass sets-up or solidifies, thus completing formation of the grains. The plastisol dispersion-sets-up or solidifies more rapidly at an elevated temperature. It has been found that a homogeneous grain can best be formed if gelatinized nitrocellulose particles and the liquid phase of the plastisol dispersion are mixed together prior to adding the modifier, but'a homogeneous grain can be obtained by other ways such as one in which the modifier is uniformly, suspended throughout the plasticizer prior to the addition of the nitrocellulose particles.

This invention further contemplates a novel powder grain having ammonium perchlorate dispersed therein. It has been found that the burning rate of nitrocellulose base powder grains can be greatly accelerated by incorporating particles of ammonium perchlorate substantially uniformly therein. Finely ground or otherwise granulated ammonium perchlorate having a maximum particle size of about 0.002 inch is preferred to insure homogeneity and good burning characteristics of the finished grain but other granulations including those within the range of .0005 to .005 inch can be utilized. Any quantity of am-' monium perchlorate of up to percent of the finished grain has been found particularly advantageous.

The particles of propellant are advantageously but notnecessarily formed by agitating nitrocellulose and a solvent therefor, such as ethyl acetate, With an excess of a non-solvent, such as water, in the presence of a protective colloid, removing the solvent, and thereafter drying the resulting pellets of nitrocellulose. Such a process is lets of high density in U.S. Patent 2,160,626.

tively free-flowing, even in the dry state, and because of their case-hardened surface characteristics which tends to resist initial attack by the solvent. This surface characteristic is undoubtedly the result of a skin eifec occasioned by surface tension and removal of the solvent in the formation of the spheres. Other ingredients, such as dioctyl phthalate, lead stearate, red lead, ethyl centralite, carbon black, dinitrotoluene, and the like, may be incorporated with the nitrocellulose in the formation of the spherical pellets to obtain particular ballistic properties as desired. Using such spherical particles, a relatively free-flowing paste is obtained in accordance with this invention when the spheres are mixed with the proper proportion of the plasticizer whereas if irregular shaped particles, such as may be obtained by merely grinding cannon powder or the like, are utilized, the desirable freeflowing characteristics of the paste are not obtained. Although the spheres of propellant may be of any desired size, large diameter spheres require relatively longer periods of time in order to elfect their solution and resultant solidification of the grain, whereas with smaller spheres, the time is correspondingly reduced. It is preferred therefore to utilize spheres having a diameter not greater than about 0.005 inch and preferably of much smaller diameter, such as 0.001 inch or less.

As indicated hereinbefore, any of the usual liquid plasticizers which are a solvent for nitrocellulose and are relatively non-volatile can be utilized to prepare the plastisol dispersion. The solvent powder of the plasticizer should be insufficient to effect any substantial solution of the nitrocellulose at ordinary temperatures within the time required to prepare the plastisol dispersion. If it is desired to prepare double base powder grains, one of the energizing modifiers such as, for example, nitroglycerine, butane triol trinitrate, diethylene glycol dinitrate, and the like, may be mixed with one or more miscible deterrents or plasticizers, such as dibutyl phthalate, dimethyl sebacate, dibutyl succinate, dibutyl adipate, triacetin, ethyl diphenyl phosphate, tributyl phosphate, and the like, to provide liquid phases which are admirably suited for the purpose. If it is desired to produce single base powder grains, the energizing modifier is omitted from the composition and the liquid phase of the plastisol dispersion is composed only of a plasticizer, such as one of those set forth above or mixtures of them. Other plasticizers suitable therefor at temperatures above their melting point are dimethyl phthalate, dioctyl sebacate, ortho nitro biphenyl, butyl benzyl phthalate, octyl diphenyl phosphate, triethylene glycol di-2-ethyl butyrate (i.e. triglycol dihexoate) and the like.

The liquid phase of the plastisol dispersion should form at least about 25 percent of the volume thereof if a relatively non-porous grain is desired. Substantially lesser amounts tend to result in voids or air pockets in the resulting powder grain and the mixture with such lesser amounts is generally not sufiiciently fluid for the purpose. Greater amounts of the liquid phase may be utilized to good advantage to increase the fluidity of the mixture, vary the ballistic properties of the resulting rocket grain as desired, and insure absence of voids or air spaces in the grains, but the amount utilized should preferably not be sufliciently great to result in segregation or settling out of the propellant and insoluble modifiers during the solution period at elevated temperature.

In operation, the liquid phase or plastisol solvent as it is commonly known is preferably desiccated or otherwise treated to reduce any traces of moisture to a minimum, and both the propellant and liquid phase are preferably evacuated prior to mixing to an absolute pressure of about ten millimeters or less of mercury for several hours. A modifier, insoluble in the plastisol solvent, such as ammonium perchlorate is then added and the dispersion intimately mixed in any suitable mixer, such as a sigma blade mixer, while the pressure of about ten millimeters or less is maintained. Such precautions as desiccating and 4 evacuating are preferably observed in order to minimiz the possibility of undesirable voids in the resulting powder grain. if the powder grain is formed by casting, the resulting dispersion is poured into asuitable mold, or otherwise shaped, in such a manner as to avoid entrapment of air and to give the desired shape of powder grain. Heat is preferably applied to elevate the temperature of the mixture to a temperature in the range of about 70 C. to C. and such temperature is maintained for a time sufiicient to cause solution of the propellant and setting-up of the mixture, at which time the temperature may be reduced. On the other hand, if the grain is to' be formed by extrusion, the mixture is heated to a temperature in the range of about 70 C. to 120 C. immediately after the foregoing mixing step and, after the propellant has dissolved in the solvent, the hot mass is forced through an extrusion die to form a length or rod of explosive, the piece being continuous as it comes from the die. The heating of the mixture may be accomplished by employing a heated worm or screw feeder for forcing the mixture through the die, with the period of dwell in the feeding device being sufiicient to heat up the mixture and cause solution of the propellant. The extruded piece is subsequently cut into suitable lengths to form homogeneous extruded powder grains. In either of the foregoing heat treatments, as a general rule, the higher the temperature utilized the shorter the period required for solution of the propellant.

In order that the invention may be further clarified, following is an example illustrating a typical embodiment thereof: Two hundred fifty parts by weight of nitrocellulose having a nitrogen content of about 12.6 percent are agitated with 4100 parts of water at about 50 C to form an aqueous slurry. A suspension of carbon black in ethyl acetate is separately prepared by first thoroughly mixing about 0.7 part of carbon black with 100 parts of ethyl acetate. This mixture of carbon black and ethyl acetate is then added to 1250 parts more of ethyl acetate, and the resultant dilute carbon black-ethyl acetate mixture is agitated to insure uniform distribution of the suspended particles of carbon black. Thirty-one and four-tenths parts of dinitrotoluene and 2.8 parts of ethyl centralite are then added to the carbon black-ethyl acetate mixture. When the dinitrotoluene and the ethyl centralite are dissolved, the resulting mixture is added to the nitrocellulose slurry while the slurry' is under agitation. The temperature is raised to 68 C. and the complete charge vigorously agitated for one hour at which time the nitrocellulose is dissolved. Twenty-five parts of a protective colloid of the type derived from animal protein dispersed in parts of water is added and the agitation is continued until the nitrocellulose solution is formed into globules of the desired small size. Vigorous agitation is required in order to form very small particles. The addition of an emulsifying agent, such as the water soluble salts of fatty acids, sulphonated oils, and so-called water-soluble oils, and the like, greatly facilitates the formation of small particles of the nitrocellulose solution, and for this reason, twenty-four parts of a 40% aqueous solution of the sodium sulphate derivative of Z-ethylhexanol is added to the bath. One hundred twenty-five parts of sodium sulfate dissolved in 300 parts of water is added over a one hour period and agitation continued for an additional three hours. The globules are then hardened to form powder grains by removal of the ethyl acetate during continuous agitation and heating up to about 99 C. in accordance with the practice set forth in US. Patent 2,027; The resultant spherical pellant such as prepared by the foregoing treating steps are placed in a sigma blade mixer and the system is evacuated to an absolute pressure of about ten millimeters of mercury and so held for about sixteen hours. Meanwhile, about parts by weight of a substantially anhydrous plastisol solvent, composed of 74% by weight nitroglycerine, 25% by weight dimethyl phthalate and 1% by weight ethyl centralite, are placed in a vessel and the vessel evacuated to an absolute pressure ofabout 10 millimeters of mercury and so maintained for aboutsixteen hours. The plastisol solvent is then added to the propellant in the mixer and about parts by weight ammonium perchlorate particles having a granulation of about 0.002 inch added thereto. The mixer is operated to intimately mix the ammoniumperchlorate, propellant and solvent, while the vacuum is maintained to prevent entrapment of air. Inasmuch as the plastisol solvent has a specific gravity of about 1.5, the percentage by volume of the liquid phase in the mixture is about 38%. After the liquid phase, ammonium perchlorate and propellant are uniformly mixed, the mixture is then carefully poured or otherwise fed into the heated worm or screw feed device, for example a Moyno pump, for passing the mixture through an extrusion die. The length of the screw feed device and the rate of discharging the mixture from the die are preferably so controlled that the mixture in traveling from one end of the screw to the other in its approach to the die is sufficiently heated to effect substantially complete solution of the propellant. It is thus possible to carry on the operation continuously. In order to accomplish this the screw feed mechanism is maintained at a temperature of about 75 C. to 120 C, with the rate of feeding thereto and discharging from the die being suitably controlled to permit solution of the propellant. The exact time of such heat treatment required may vary from a very few minutes up to several hours and depends upon the solvent activity of the plastisol solvent, the temperature of the mixture, the particle size of the propellant, and the efiective resistance of the surface of the propellant grains, hereinbefore described as case hardening, but in any event the mixture is maintained at an elevated temperature for a sufficient length of time to effect a solution of the propellant and sulficient setting up of the mass to permit formation of the desired powder grains. As the gelatinized mass is extruded from the die it is cut into powder grains of the desired length.

In another embodiment of this invention, about 5 parts by weight propellent pellets made in accordance with the process described in detail in connection with the foregoing embodiment are placed in a suitable mixer and the system is evacuated to an absolute pressure of about 10 millimeters of mercury and so held for about 72 hours. Simultaneously, about 10 parts by weight plastisol solvent composed of about 49.5 percent nitroglycerine, 49.5 percent glycerol triacetate and 1 percent by weight 2-nitrodiphenylamine are placed in a vessel and the vessel evacuated to an absolute pressure of about 10 millimeters of mercury and so maintained for about 72 hours. After this treament, the plastisol solvent is added to the propellant in the mixer, about 15 parts ammonium perchlorate is added to the resultant mixture and the mixer operated to intimately mix the propellant, plastisol solvent or liquid phase and amonium perchlorate and to disperse the propellant and ammonium perchlorate sub stantially uniformly throughout the solvent to form a plastisol dispersion, while the vacuum is maintained to prevent entrapment of air. After the plastisol dispersion is formed, the mixture is shaped by carefully pouring to avoid entrapment of air into a mold formed of nitroglycerine-resistant plastic such as cellulose acetate, methyl methacrylate or ethyl cellulose. .Inasmuch as the solvent has a specific gravity of about 1.29, the percent by volume of plastisol in the mixer is about 41.8.

The mold containing the plastisol dispersion is then placed in an oven, heated to about 75 C. and maintained at this temperatureuntil the propellant is substantially dissolved by the solvent and the grain has set up. This solution and setting up of the grainis readily accomplished in a relatively short time, but for instance, may be conveniently let set over night. The exact time of such heat treatment depends, of course, upon the activity of the solvent, the particle size of the propellant, and the effective resistance of the surface of the propellant, grains, hereinbefore described as case hardening. For instance, utilizing the propellant and plastisol solvent described above, the mixture, if let stand at the relatively low temperature of about 22 C. will ordinarily set up in about 21 hours. When the grain has set up it is permitted to cool to room temperature and is removed from the mold.

Powder grains prepared in accordance with this invention are substantially free of undesirable voids and are substantially uniform in composition throughout the grain insuring the constant as well as rapid burning rate. It will be apparent that the method is relatively simple primarily because the grains are formed from a mobile paste and that large homogeneous powder grains containing an insoluble modifier as well as a modifier soluble in the liquid phase and having a diameter of from one-half inch to several inches and of any length up to several feet as well as smaller grains can be made in accordance with this invention. Furthermore, it will be apparent that the solvent, propellant and modifier can be varied appreciably in the composition and that this invention provides powder grains having substantially any ballistic characteristic including powder grains containing ammonium perchlorate having burning rates above that of conventional powder grains. In addition, the use of volatile solvents is avoided.

While the foregoing embodiments have been described in detail, it is to be understood that many modifications and variations will present themselves to those skilled in the art without departing from the spirit of this invention or the scope of the appended claims.

Having thus described the invention, what is claimed and desired to be secured by Letters Patent is:

1. A plastisol dispersion consisting essentially of at least 25 percent liquid substantially non-volatile plasticizer and solvent for nitrocellulose and, substantially uniformly dispersed in said plasticizer, up to 65 percent particles of a solid modifier of the ballistic characteristics of a propellent powder selected from the group consisting of lead azide, ammonium perchlorate, lead, lead stearate, lead oxide, iron, lead sulfide, lead chromate, aluminum dust and ammonium nitrate, and particles of gelatinized nitrocellulose substantially insoluble in said plasticizer at the temperature at which said dispersion is made but soluble therein at higher temperatures.

2. The plastisol of claim 1 wherein said plasticizer is composed of about 74 percent nitroglycerine, about 25 percent non-explosive plasticizer and about 1 percent ethyl centralite and the gelatinized particles of nitrocellulose are spheres having a diameter not greater than about 0.005 of an inch.

3. A solid substantially homogeneous gelatinized nitrocellulose base propellent powder grain having dispersed therein up to 65 percent particles of ammonium perchlorate having a granulation within the range of 0.0005 inch to 0.005 inch.

4. A solid gelatinized nitrocellulose base propellent powder grain having a diameter of at least one-half inch and having up to 65 percent ammonium perchlorate having a granulation within the range of 0.0005 inch to 0.005 inch dispersed substantially uniformly throughout.

5. A solid substantially homogeneous gelatinized nitrocelluose base propellent powder grain having a diameter of at least about one-half inch and containing up to 65 percent ammonium perchlorate having a. granu- 7 lation within the range of 0.0005 inch to 0.005 inch and with the balance being nitrocelluose and a nitropolyhydric alcohol.

References Cited in the file of this patent UNITED STATES PATENTS 1,510,555 Olsen Oct. 7, 1924 1,627,691 Du Pont et a1 May 10, 1927 2,027,114 Olsen et a1. Jan. 7, 1936 

1. A PLASTISOL DISPERSION CONSISTING ESSENTIALLY OF AT LEAST 25 PERCENT LIQUID SUBSTANTIALLY NON-VOLATILE PLASTICIZER AND SOLVENT FOR NITROCELLULOSE AND, SUBSTANTIALLY UNIFORMLY DISPERSED IN SAID PLASTICIZER, UP TO 65 PERCENT PARTICLES OF A SOLID MODIFIER OF THE BALLISTIC CHARACTERISTICS OF A PROPELLENT POWDER SELECTED FROM THE GROUP CONSISTING OF LEAD AZIDE, AMMONIUM PERCHLORATE, LEAD, LEAD STEARATE, LEAD OXIDE, IRON, LEAD SULFIDE, LEAD CHROMATE, ALUMINUM DUST AND AMMONIUM NITRATE, AND PARTICLES OF GELATINIZED NITROCELLULOSE SUBSTANTIALLY INSOLUBLE IN SAID PLASTICIZER AT THE TEMPERATURE AT WHICH SAID DISPERSION IS MADE BUT SOLUBLE THEREIN AT HIGHER TEMPERATURES. 